Bone and soft tissue tumors (BSTTs) are important yet understudied entities in human oncology. Our incomplete understanding of their pathogenesis has limited development of effective therapies. This proposal focuses on the role of TRE17/ubiquitin-specific protease 6 (USP6), in BSTT pathogenesis. Recurrent translocations of TRE17 occur in two BSTTs, aneurysmal bone cyst (ABC) and nodular fasciitis (NF), resulting in overexpression of wild type TRE17. TRE17 overexpression also arises selectively in other BSTTs, particularly the highly lethal alveolar rhabdomyosarcoma (ARMS). TRE17 potently activates the transcription factors NFkB and STAT3, in a manner dependent on its USP activity. Both NFkB and STAT3 are frequently dysregulated in cancer, often coordinately, to promote tumor cell survival and proliferation, inflammation, and angiogenesis. This proposal will test whether NFkB and STAT3 are critical cellular effectors of TRE17 in the pathogenesis of ARMS and ABC/NF. Aim 1: Determine role of Jak1/STAT and NFkB pathways in TRE17- mediated proliferation and survival. TRE17's USP activity is essential for tumorigenesis, and preliminary data implicate Jak1, the STAT3 kinase, as its first substrate. Exciting new results show that Jak1 can be modified by ubiquitin and the Ub-like molecule, ISG15, and suggest TRE17 may counteract both of these modifications to induce its stabilization. Cell culture-based studies will be used to test this, and to determine whether Jak1/STAT3 and NFkB are essential for TRE17-mediated proliferation, survival, and induction of modulators of the tumor microenvironment. Aim 2: Determine requirement for Jak1/STAT and NFkB in TRE17-mediated tumorigenesis in murine models of ABC/NF. We have developed murine xenograft models of ABC and NF, tumors driven by TRE17 translocation. We will examine whether NFkB/Jak1/STAT3 are required cell- autonomous requirement for tumor cell survival and proliferation, and determine their role in modulating the inflammatory and angiogenic microenvironment using immunohistochemistry and in vivo bioluminescence imaging. Aim 3: Determine role of TRE17 and NFkB/STAT3 in ARMS pathogenesis. ARMS is a highly malignant BSTT with dismal survival rates. Through analysis of patient-derived ARMS cell lines and primary tumor samples, we will determine whether TRE17 is an essential mediator of ARMS pathogenesis, and whether it functions through Jak1/STAT3 and NFkB. We will examine the requirement of these factors in growth and proliferation in vitro, including their role within ARMS cancer stem cells. Transcriptome analysis will be performed to define a TRE17-response signature. Finally, we will determine the roles of TRE17, Jak1/STAT3 and NFkB in ARMS tumor formation, maintenance, and metastasis in mice. These studies may ultimately lead to novel approaches (TRE17-specific USP inhibitors, and NFkB and Jak1/STAT inhibitors) for the treatment of lethal cancers such as ARMS, and other BSTTs in which TRE17 is overexpressed.